The present invention relates to housing technology. In particular, the present invention relates to a housing and to a method for closing a housing by means of an optical joining method, such as laser welding, by way of example. In addition, the present invention also relates to a platform, particularly a satellite, having a housing according to the invention, as well as to the use of the method according to the invention for the purpose of manufacturing a housing.
Various different application scenarios are known, such as those in which electronic assemblies are arranged in a hermetically sealed housing in applications involving vacuums, high pressure, or low-pressure, by way of example. Suitable electrical conductors provide an electrical connection from the components arranged in the housing to the outside. Such hermetically sealed housings are used, by way of example, in satellite applications, including hybrid circuits and/or microwave modules or opto-electrical hybrid modules for satellite communication applications, for example.
A further aspect in this context is the hermetical closure of a housing after a circuit has been inserted into the housing. Suitable methods of closure include laser welding, oven soldering and/or reflow soldering, as well as resistance soldering and/or resistance welding, by way of example.
In the case of laser welding, the lid and the edge of the housing, of a housing, are heated very locally by the laser beam until the metal melts. Because the applied energy is comparably small at several Joules per weld point, the component is only subjected to a low heat load overall.
In oven soldering, the housing is heated together with the lid to a soldering temperature, such that a solder connection is formed between two metal surfaces. In this case, the component undergoes high thermal loading as a whole.
In resistance soldering and/or welding, energy in the form of a current is applied to the housing and/or housing elements using electrode rolls. With this type of contact, the transition resistance between an electrode and a housing element can vary greatly, thereby increasing the probability of local errors in the solder. The application of energy to the housing elements in the case of resistance soldering and/or welding is in the range between that of laser welding and oven soldering.
Whereas in the case of laser welding, it is possible to join a metal housing in a very hermetic manner, in the case of ceramic materials, the high pulse power of the laser pulse used, at several kW, can lead to cracks in the ceramic material.
According to the invention, both a housing and a method for closing a housing are disclosed in which the connection is created using an optical joining method such as a soldering process utilizing a laser, for example, and particularly an infrared laser, and in which the unrestricted dissipation of heat in the housing is limited by a local reduction in heat conductivity.
Accordingly, a housing, a method for closing a housing, a platform, particularly a satellite, having a housing according to the invention, and also the use of the method according to the invention for the purpose of manufacturing a housing are disclosed.
According to one exemplary embodiment of the present invention, a housing has a first housing element and a second housing element, wherein the housing elements have a heat conductive design, and wherein the housing elements can be joined by means of a joining process, wherein a joining material is configured between the first housing element and the second housing element for the purpose of joining the housing element, and wherein at least one of the housing elements can be heated such that the joining material is conditioned for the joining of the housing elements, wherein at least one of the housing elements has a local reduction in heat conductivity such that the dissipation of heat in the at least one housing element is reduced.
According to a further exemplary embodiment of the present invention, a method for closing a housing is disclosed, including the provision of a first housing element and the provision of a second housing element, wherein the housing elements have a heat conductive design, and wherein the housing elements can be joined by means of a joining process, the configuration of a joining material between the first housing element and the second housing element for the purpose of joining the housing elements, and the heating of at least one of the housing elements such that the joining material is conditioned for the joining of the housing elements, wherein at least one of the housing elements has a localized reduction in the heat conductivity thereof such that the dissipation of heat in the at least one housing element is reduced.
According to a further exemplary embodiment of the present invention, a platform is disclosed, particularly a satellite, having a housing according to the invention.
According to a further exemplary embodiment of the present invention, the method according to the invention is used for the production of a housing.
The continuing development of hybrid assemblies leads to housings which are ever-smaller, because—among other things—the electronic components accommodated in the same are undergoing a constantly-advancing miniaturization. A subsequent size reduction in the housing, in this case, places increasingly greater requirements on a closure method by means of which a housing is hermetically closed—for example by means of a cover element—because increased temperatures arise on and/or in the housing in smaller housings during the joining process by means of heating the joints and joining regions, and this can be problematic for the electronic components accommodated therein.
Particularly for housing elements made of a ceramic material, which has a comparably good ability to conduct heat, a comparably large amount of heat—for example from a cover element which is irradiated with laser light, and from a solder point positioned beneath the same—can flow into the ceramic housing, wherein this heat energy can then be transferred to electronic components in the housing.
For this reason, a joining process that is an alternative to a laser welding process is required in all housing components—for example ceramic housings—wherein a localized and/or average thermal load must be kept as minimal as possible. In addition, a housing may be required which at least reduces the transmission of heat energy in the housing during a joining process.
Such a novel housing, as well as such a novel joining method—for example designed as a laser soldering method—are provided by the present invention.
In the case of laser soldering, a joining material, for example a solder material, is melted between two housing elements. This may be carried out with a laser in the infrared wavelength region, by way of example, the laser applying its light to the upper side of a housing element—typically the cover element. By means of a quick scan and/or movement along a desired contour or joining seam of a housing element above the solder, sufficient energy is applied to the housing element in this case to liquefy a solder positioned in or on a housing element, and thereby melted. The joining material can subsequently suitably join the housing elements such that a hermetically sealed housing is formed. The housing element in this case can consist of a metal or ceramic material.
According to a further preferred embodiment of the present invention, at least one of the housing elements is constructed from a metal or a ceramic material, particularly a ceramic material with a metal coating. According to a further exemplary embodiment of the present invention, the housing can have a hermetically sealed construction following the completion of the joining process.
As a result of the transfer of heat by the solder to a housing edge surface of the housing element below, a solder joint is achieved which is particularly hermetically tight after it cools.
Pulsed laser soldering particularly constitutes a comparably very low thermal load for housing elements. However, this thermal load can be thermally critical for small and extremely small housings. For safety reasons, it is necessary to prevent the transmission of heat via the ceramic housing to components in the interior space. The term “transmission of heat” in this case particularly indicates the transfer of heat energy beyond the joint and/or solder seam into the housing, and to the components arranged therein. By means of suitable constructive measures, however, it is possible to prevent, and/or at least limit, the migration of heat into undesired regions of a housing, such that a housing remains relatively cool. In any case, the actual soldering time is significantly shorter than in an oven process—particularly because the heat transfer ends substantially instantaneously once the laser is switched off.
According to a further preferred embodiment of the present invention, a temperature barrier is included in at least one of the two housing elements, particularly at least in the housing element that is not heated, as a localized reduction of the heat conductivity thereof, wherein the temperature barrier can be designed as a localized reduction in the thickness and/or a localized cross-section narrowing of a housing wall.
In this case, temperature barriers that are suitable constructive measures are those which are included on ceramic walls of a housing element, by way of example, and particularly the housing element that is not irradiated by the laser. Such temperature barriers can be designed as localized reductions and/or constrictions of the thickness of a housing wall. In this way, the heat conductivity of a housing element is reduced, and particularly locally, such that the transfer of applied heat energy past these temperature barriers is delayed, and therefore is impeded. An additional advantage is that a heat energy bottleneck occurs in the region of a housing element that is arranged prior to the temperature barrier when viewed from the perspective of the laser, thereby favoring the heating of the region before the temperature barrier, by means of the applied laser energy.
According to a further exemplary embodiment of the present invention, the joining material is designed as a means found in the group consisting of solder, solder paste, solder wire, and solder preform, and particularly the joining material is free of fluxing agent.
If a solder joining process is mentioned in the field of aerospace applications, the term typically indicates soldering without fluxing agent. In a solder process that does not use fluxing agent, ideally no contaminants are formed needing to be removed following the joining process at great expense of time and effort.
For soldering processes without fluxing agent, the surfaces being joined must be wettable, meaning that there can neither be any visible contaminants, nor can the surface or the layer beneath the same be highly oxidized and/or contaminated with foreign atoms. The latter typically occurs when a housing element is stored improperly—for example in moist indoor air rather than in a dry nitrogen cabinet.
Contamination by foreign atoms in this case is difficult to demonstrate, and is generally only recognized during a soldering process when the solder material flows poorly, because the surface has poor wettability. In order to limit this, a joining process is typically carried out in a forming gas—a nitrogen/hydrogen mixture with 5-10% hydrogen, and in the worst case scenario, with 100% hydrogen. The hydrogen in this case has a reducing and/or oxygen-binding effect that is otherwise provided by a fluxing agent. The use of forming gas with a small fraction of hydrogen does not require any special safety precautions, while significant safety precautions must be made when pure hydrogen is used.
Laser soldering is a suitable method for reliably joining two electrical contacts to each other without thermally loading neighboring components. The joining material and/or solder can be applied to the joint position by an automatic solder wire feed, a suitably applied solder paste, or by a solder preform. Laser soldering is known, for example, in the assembly of electronic SMD circuit boards and/or in the assembly of heat-sensitive electronic assemblies, and in fully-automatic joining technology used in electromechanical components such as switches or motors, by way of example.
According to the invention, two housing elements are soldered, by their suitable surfaces, using a preferably fluxing agent-free joining material and/or solder, by means of an optical joining process such as laser soldering, by way of example. A laser beam is used for the purpose of heating the solder to the melting temperature, as well as heating the surfaces being joined—for example the housing edge and lid—to the soldering temperature. In this case, it is particularly preferred that metallic surfaces are joined to each other. However, as an alternative, the method according to the invention can also be carried out for housing elements made of a ceramic material.
According to a further preferred embodiment of the present invention, the heated housing element has a conditioned surface to facilitate the application of optical energy thereto; in particular, the surface of the housing element has a modified surface and/or an altered degree of reflection of the surface at least locally in the region in which the optical energy is applied.
Metallic surfaces in particular generally have a high degree of reflection, with the result that a significant fraction of incident laser light is reflected. One aspect of the present invention is the modification of a degree of reflection of a housing element surface. The surface near the solder joint can be modified in a targeted manner such that an acceptable absorption of the laser light can be achieved. A modification in this case can be that a surface plating, and therefore a coating of the housing surface that is commonly made of gold on nickel, is removed and/or the surface is roughened. For this as well, a suitable laser can be used which is able to execute an ablation of the surface. It is particularly preferred in this case that the same laser element is used as for the joining process—and potentially is controlled with modified parameters.
A modification of the surface in this case is not necessary over the entire housing element surface; the localized region of the solder joint is sufficient.
According to a further preferred embodiment of the present invention, at least one housing element is routed to a prespecified track during the application of the optical energy. According to a further preferred embodiment of the present invention, a laser element is included, both to condition the surface for the application of the optical energy to the same, and to apply the joining energy itself.
For the purpose of achieving an even heating of the surfaces being soldered, the heat energy can be applied to a housing element by means of a laser scanner, and therefore as a controllable laser beam, in a contactless manner, in the shape of the housing-lid contour. Such a housing-lid contour in this case substantially corresponds to the contour of the desired connection between the housing elements, and generally has a rectangular design—although other shapes can also be contemplated which are nonetheless based on a concrete housing construction. The laser light is absorbed by a surface of a housing element, which provides the conductance of the heat energy by the housing element to the joining material and/or solder. A further conductance of the heat energy by the solder to the second housing element being soldered enables the melting of the solder after a certain warm-up time, and therefore enables the production of a mechanically strong and hermetically sealed connection between the two housing elements, by means of the melted solder material.
It is particularly preferred that solders are used with no fluxing agent, such that it is not necessary to clean the housing elements following the joining process. So that it is possible to reliably use fluxing agent-free solders, an activation of the surface being soldered is generally necessary. An activation in this case can take place by baking the housing elements, typically a housing and a lid, in a vacuum oven, for example for the purpose of removing moisture by means of scrubbing with forming gas at elevated temperature, and optionally in combination with intermediate vacuum cycles. As an alternative, the joining process can be carried out using fluxing agent-free solder itself in forming gas and/or a nitrogen/hydrogen mixture.
According to a further preferred embodiment of the present invention, it is possible to further monitor the processing temperature when the two housing elements are joined, particularly by means of an IR camera and/or a temperature sensor.
To increase the reliability of the process, a so-called in-process control can be carried out during the solder process as part of the joining process. This can be realized, by way of example, by an infrared camera, an infrared temperature sensor, or another suitable temperature measurement which is substantially able to test whether the region in which the joining process will join the two housing elements has a suitable process temperature which will lead to the desired result.
According to a further preferred embodiment of the present invention, the two housing elements can be fixed to each other in their relative position during the joining process, by means of a fixing device, particularly a mechanical press device, a weight or a magnet system, and more particularly a self-centering magnet system.
In addition, a suitable mechanical fastening can be configured in a housing element, such that an upper solder partner, by way of example—which is typically a lid with a solder preform—does not float and shift its position on the melted and therefore fluid joining material during a solder process. As such, a weight element, a magnet element, particularly a self-centering magnet, and/or a housing element that is a magnet element which is self-centering relative to the further housing element, can be configured on this housing element. In addition, a mechanical press device, particularly outside of the laser focus through which a laser beam must be guided, can press a housing element with a constant force against the further housing element, and/or hold down the same on said further housing element.
According to a further preferred embodiment of the present invention, the optical joining method can be designed as an optical soldering process, particularly a laser soldering process. According to a further preferred embodiment of the present invention, the joining process can be carried out using energy applied optically.
The joining process and/or laser solder process according to the invention has good reproducibility at high yields, and particularly demonstrates low thermo-mechanical stress in metallized ceramic housings. In laser soldering, heat energy is applied to a very limited local area by means of light, and therefore optically.
The provision of a suitable deflection device for the laser beam, for example a rapidly adjustable galvanic mirror in the kH range, enables a nearly arbitrary routing contour for the laser beam on a housing element, such that ideally only the region of the joining material and/or solder is fully heated as a whole, and liquefies essentially simultaneously.